Thin-film photovoltaic (PV) devices may be used to create solar cells, detectors, electronic devices, telecommunication devices, charge-coupled imaging devices (CCDs), computers, and even biological or medical devices (together considered “thin-film compound semiconducting materials”). With regard to renewable energy, solar cells are devices that have characteristics that enable them to convert the energy of sunlight into electric energy. The aim of research often is to achieve solar cell designs with the lowest cost per watt generated by the solar cell, and, concurrently, the designs should provide solar cells that are suitable for inexpensive commercial production.
The potential market for thin-film photovoltaic (PV) devices is enormous and is expected to continue to grow in the coming years. Recently, a goal was set to globally deploy one terawatt of continuous PV-based power, and achieving this goal will require an industry that can supply on the order of 200 gigawatt peak (GWp) of PV modules each year. Additionally, in the United States, goals concerning costs have been set that include a module-level cost goal for utility-scale PV installations of 0.5 $/Wp, which would make unsubsidized PV competitive with conventional power sources. At this cost level and at a deployment level in the hundreds of GWp per year, PV module sales globally may be in excess of $50 billion (in U.S. dollars) per year. As will be appreciated, any technology that can better enable the PV industry, such as by increasing efficiencies, reducing material costs, lowering manufacturing expenses, and the like, has a large potential for growth and revenue generation.
A conventional thin-film solar cell is composed of a stacking of thin layers on a substrate, in which the thin layers form one or more junctions with differing band gaps that absorb light and convert it into electricity. Presently, most compound semiconductor thin-film solar cells are fabricated with an absorber, or absorber layer formed of cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS), both of which have high optical absorption coefficients and have versatile optical and electrical characteristics. The thin-film PV industry based on CdTe and CIGS recently had sales approaching $2 billion in U.S. dollars) per year, with sales continuing to rapidly grow.
Unfortunately, there are concerns in the PV industry that reliance on CdTe and CIGS thin-film devices may not be sustainable. For example, many have expressed concern that the costs of raw materials such as tellurium and indium will increase as the PV industry continues to grow and demand for these commodities or raw materials will someday outstrip supply. When this happens, there will inevitably be upward forces on production costs that will both curtail growth of the thin-film PV industry and prevent thin-film PV from achieving the above-stated goals including a goal of 0.5 $/Wp. Additionally, both indium and tellurium have use in non-PV-related industries that are not subject to the severe cost restraints that face thin-film PV, and these non-PV uses may generate further price pressures on these raw materials.
The concerns over the availability and cost of raw materials has caused many in the PV industry to attempt to develop thin-film PV devices that use materials that are based on abundant and less toxic elements for the thin-film layers including the absorber layer. For example, Cu2ZnSn(S,Se)4 or CZTSS is a compound semiconductor material that has recently been under investigation in the PV industry for use in thin-film PV devices. For example, CZTSS thin films are p-type and may serve as an absorber layer in thin-film solar cells in place of a thin film of CdTe or CIGS. CZTSS is structurally and chemically similar to more well-known chalcopyrite materials such as CIGS, and CZTSS displays many of the same attributes that have made CIGS a successful material for high-efficiency (i.e., greater than 20 percent efficiencies) thin-film PV devices.
In 2010, a thin-film PV device with a CZTSS absorber was fabricated with an efficiency of 9.7 percent. Such efficiency is remarkably high considering how little research has been done on CZTSS materials by the PV industry. The tested CZTSS film had several characteristics that may have lowered that achieved efficiency. For example, cross-sectional SEM images indicated that the film had a morphology that included numerous voids and that the grain size was relatively small at less than about 2 microns. While CZTSS films may someday be used commercially as an absorber in PV devices, the PV industry may demand processes for fabricating higher quality CZTSS films such as films with less voids, with selectable grain sizes (e.g., larger grains to limit losses associated with charge carriers crossing grain boundaries), and with more desirable optoelectronic properties.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.